1. Field of the Invention
The present invention relates to a vacuum interrupter that performs interruption/conduction of current in vacuum, and to a vacuum switch wherein this vacuum interrupter is mounted, more particularly, it relates to improvements in the contact resistance characteristic and restriking characteristic of the contacts of the vacuum interrupter.
2. Description of the Related Art
In order to maintain or improve, apart from the three fundamental requirements typified by the anti-welding characteristic, voltage withstanding characteristic and interruption characteristic, the current chopping characteristic, erosion characteristic, contact resistance characteristic and temperature rising characteristic etc., the contacts of vacuum interrupters mounted in a vacuum switch or vacuum circuit breaker are constituted of various base materials. However, it is considered to be impossible for these to be fully satisfied by a single element, since the above required characteristics often demand mutually contradictory material properties.
Accordingly, contact materials have been developed for specific applications such as large current interruption applications or high withstand-voltage applications, by use of composite materials or by base material cladding etc., and these exhibit excellent characteristics in their own way. For example, as contact materials for large current interruption satisfying the three fundamental requirements, there are known Cuxe2x80x94Bi alloys, or Cuxe2x80x94Te alloys containing 5 weight % or less of anti-welding constituents such as Bi or Te (Issued Japanese patent Sho. 41-12131, and Issued Japanese patent number Sho. 44-23751).
Cuxe2x80x94Bi alloy has excellent large-current interruption characteristics, since a low welding separation force is achieved by the embrittlement of the alloy itself which is produced by the presence of brittle Bi segregated at grain boundaries. Likewise, Cuxe2x80x94Te alloy has excellent large-current interruption characteristics, since a low welding separation force is achieved by the embrittlement of the alloy itself which is produced by the presence of brittle Cu2Te segregated at grain boundaries and inner grains.
In contrast, Cuxe2x80x94Cr alloy is known as a contact material for high withstand-voltage/large current interruption use. This alloy has a smaller vapor pressure difference between its structural constituents than have the aforementioned Cuxe2x80x94Bi alloy or Cuxe2x80x94Te alloy, and so has the advantage that it can be expected to exhibit uniform performance, and indeed is excellent, depending on the application. Cuxe2x80x94W is also known as a high withstand-voltage contact material. These alloys exhibit excellent anti-arcing characteristics, on account of the effect of the high melting point materials.
In a vacuum circuit breaker and/or vacuum switch, the phenomenon may be induced that, after current interruption, flashover occurs within the vacuum interrupter, causing a conductive condition between the contacts to be re-established (subsequent discharge does not continue). This phenomenon is called the restriking phenomenon, but the mechanism of its occurrence has not yet been elucidated. Abnormal over-voltages frequently occur on account of the rapid change to a conductive condition after the electrical circuit was first put in the current-interrupted condition. In particular, in tests wherein restriking was produced on interruption of a condenser bank, the occurrence of extremely large over-voltages and/or excessive high-frequency current was observed. The development of a technique for lowering the probability of restriking is therefore sought.
Although, as described above, the mechanism of occurrence of the restriking phenomenon is not known, according to the experimental results of the inventors, restriking occurs with fairly high frequency between one contact and another contact or between the contacts and the arc shield within the vacuum interrupter. Accordingly, the inventors succeeded in greatly reducing the number of occurrences of restriking by discovering that techniques for suppressing abrupt gas that is discharged for example when the contacts are subjected to arcing and techniques for optimization of the contact surface condition are extremely effective in lowering the probability of restriking.
In recent years, however, to meet demands for improving the voltage withstanding performance and demands for improving the large current interruption performance of vacuum interrupters, in particular demands for miniaturization, further reductions in restriking of the contacts are required. Specifically, in recent years, severity of the conditions of use demanded by users and of the variety of loads have increased. A marked recent trend is increasingly frequent application to reactor circuits and capacitor circuits. The development and improvement of contact materials for this has become an urgent task.
In the case of capacitor circuits, about two or three times of the usual voltages are applied, so the surface of the contacts is severely damaged by arcing during the current interruption or current switching, and, as a result, surface roughening and exfoliative erosion of the contacts is promoted. Such surface roughness and/or exfoliation increases contact resistance, and is believed to be a factor causing restriking. Thus, although it is unclear which is the initial trigger, cause and effect are repeated, with the result that the frequency of occurrence of restriking and the contact resistance both increase. However, notwithstanding the importance of the phenomenon of restriking from the point of view of product reliability, and neither a way of preventing it nor its direct causes have yet been elucidated.
When the inventors observed in detail the correlation with occurrence of restriking of the total quantity of gas discharged in the heating step of Cuxe2x80x94W alloy or Cuxe2x80x94Mo alloy, the type of gas and its mode of discharge, they discovered that, in the case of contacts where there was considerable abrupt discharge of gas in pulse fashion in the vicinity of the melting point, albeit for a very short time, the rate of restriking was also high.
Accordingly, the restriking phenomenon was reduced by subjecting the Cu, W raw material or Cu, Mo raw material or Cuxe2x80x94W contact alloy or Cu, Mo contact alloy beforehand to heating in the vicinity of the melting temperature or above the melting temperature, or removing beforehand factors causing the discharge of abrupt gas in the Cuxe2x80x94W alloy or Cu, Mo contact alloy, or high temperature aging of the Cuxe2x80x94W contact surface layer or Cuxe2x80x94Mo contact surface layer or by improving sintering techniques so as to suppress pores and/or structural segregation in the Cuxe2x80x94W alloy or Cuxe2x80x94Mo alloy.
However, with the further demands for suppression of restriking in recent years, the need for further improvements has been recognized and in particular development of other strategies has become important.
As described above, for high withstand-voltage contact materials, Cuxe2x80x94W alloy or Cuxe2x80x94Mo alloy were used in preference to the Cuxe2x80x94Bi alloy, Cuxe2x80x94Te alloy or Cuxe2x80x94Cr alloy described above, but in fact they cannot be described as contact materials that can fully meet the increasingly severe requirements for reduction of restriking. Specifically, even in the case of Cuxe2x80x94W alloy or Cuxe2x80x94Mo alloy which have been preferentially used hitherto, occurrence of restriking in more severe high voltage regions and in circuits where there is rush current, or the existence of instability of the contact resistance characteristic caused by the material properties of the Cuxe2x80x94W alloy or Cuxe2x80x94Mo alloy have been identified as problems.
Accordingly, the development of contact material for vacuum interrupters having in particular excellent restriking characteristics and contact resistance characteristics, while still maintaining a certain level of the aforementioned fundamental three requirements, is desired.
Accordingly, one object of the present invention is to provide a novel vacuum interrupter and vacuum switch in which this is mounted, comprising contacts whose contact resistance characteristic and restriking characteristic can be simultaneously improved, by optimizing the metallurgical conditions of the Cuxe2x80x94W alloy or Cuxe2x80x94Mo alloy.
In order to achieve the above object, in a vacuum interrupter that performs current interruption/conduction by opening/closure of contacts in vacuum, the contacts referred to above are manufactured of contact material constituted by, as anti-arcing constituent, W of mean grain size 0.4 to 9 xcexcm and 65 to 85 weight %, as restriking stabilization auxiliary constituent, 0.09 to 1.4 weight % of CuxSb chemical compound, and, as conductive constituent, Cu or CuSb alloy as the balance.
If the mean grain size of the W exceeds 6 xcexcm, uniform dispersion of the CuxSb chemical compound is impeded. If this is less than 0.4 xcexcm, there is a considerable amount of gas left in the base material, which is undesirable for contact material. If the W content is in the range 65 to 82 weight %, the contact resistance characteristic and restriking characteristic coexist in a desired range. If the W content is more than 82 weight %, the contact resistance characteristic is impaired, while if the W content is less than 70 weight % the restriking characteristic is impaired. If the content of CuxSb chemical compound is in the range 0.09 to 1.4%, the contact resistance characteristic and restriking characteristic coexist in a desired range. If the content of CuxSb chemical compound is more than 1.4%, the contact resistance characteristic and restriking characteristic are both adversely affected. If the content of CuxSb chemical compound is less than 0.09%, control of the Sb content in the contacts alloy is difficult, a uniform dispersion and distribution of the Sb constituent at the contact surface is not obtained, and the contact resistance characteristic and restriking characteristic are both adversely affected.
Furthermore, in a vacuum interrupter that performs current interruption/conduction by opening/closure of contacts in vacuum, the contacts referred to above are manufactured of contact material constituted by, as anti-arcing constituent, in integrated form and size in the range 0.4 to 10 xcexcm, W of mean grain size 0.4 to 9 xcexcm and 65 to 85 weight % and Mo of mean grain size 0.4 to 9 xcexcm of 0.001 to 5 weight % and as restriking stabilization auxiliary constituent, 0.09 to 1.4 weight % of CuxSb chemical compound, and, as conductive constituent, Cu or CuSb alloy as the balance.
The presence of a prescribed small content of Mo improves the plastic deformation capability of W in regard to thermal or mechanical shock to which the W is subjected during circuit braking action or switching action, and thus has the benefit of suppressing chipping of W in extremely minute, micro-scale portions. It therefore contributes to reduction of in particular the range of variability of the frequency of occurrence of restriking. If the Mo content exceeds 5 weight %, its benefit is lessened.
Yet further, in a vacuum interrupter that performs current interruption/conduction by opening/closure of contacts in vacuum, the contacts referred to above are manufactured of contact material constituted by, as anti-arcing constituent, Mo of mean grain size 0.4 to 9 xcexcm and 50 to 75 weight %, as restriking stabilization auxiliary constituent, 0.09 to 1.4 weight % of CuxSb chemical compound, and, as conductive constituent, Cu or CuSb alloy as the balance.
If the mean grain size (diameter) of the Mo exceeds 9 xcexcm, uniform dispersion of the CuxSb chemical compound is impeded. If this is less than 0.4 xcexcm, there is a considerable amount of gas left in the base material, which is undesirable for contact material. If the Mo content is in the range 50 to 75 weight %, the contact resistance characteristic and restriking characteristic coexist in a desired range. If the Mo content is more than 75 weight %, the contact resistance characteristic is impaired, while if the Mo content is less than 50 weight % the restriking characteristic is impaired. If the content of CuxSb chemical compound is in the range 0.09 to 1.4%, the contact resistance characteristic and restriking characteristic coexist in a desired range. If the content of CuxSb chemical compound is more than 1.4%, the contact resistance characteristic and restriking characteristic are both adversely affected. If the content of CuxSb chemical compound is less than 0.09%, control of the Sb content in the contacts alloy is difficult, a uniform dispersion and distribution of the Sb constituent at the contact surface is not obtained, and the contact resistance characteristic and restriking characteristic are both adversely affected.
Yet further, in a vacuum interrupter that performs current interruption/conduction by opening/closure of contacts in vacuum, the contacts referred to above are manufactured of material constituted by, as anti-arcing constituent, in integrated form and size in the range 0.4 to 10 xcexcm, Mo of mean grain size 0.4 to 9 xcexcm and 50 to 75 weight % and W of mean grain size 0.4 to 9 xcexcm and 0.001 to 5 weight % and as restriking stabilization auxiliary constituent, 0.09 to 1.4 weight % of CuxSb chemical compound, and, as conductive constituent, Cu or CuSb alloy as the balance.
The presence of a prescribed small content of W (forming MoW in integrated form with Mo) improves the plastic deformation capability of Mo in regard to thermal or mechanical shock to which the W is subjected during circuit braking action or switching action, and thus has the benefit of suppressing chipping of Mo occurring at the contact surface in extremely minute, micro-scale portions. It therefore contributes to reduction of in particular the range of variability of the frequency of occurrence of restriking. If the W content exceeds 5 weight %, its benefit is lessened.
In another preferred mode of the present invention, the CuSb alloy referred to above contains in solid solution less than 0.5 weight % of Sb.
CuSb alloy containing more than 0.5 weight % of Sb in solid solution has severely impaired conductivity and cannot be utilized for contact material.
In another preferred mode of the present invention, the x in the chemical compound CuxSb referred to above is x=1.9 to 5.5.
If the ratio x in regard to the Cu is outside the range 1.9 to 5.5, smoothness of the contact surface is difficult to obtain.
In another preferred mode of the present invention, the chemical compound CuxSb referred to above may be any one or more selected from the group consisting of: Cu5.5Sb, Cu4.5Sb, Cu3.65Sb, Cu3 5Sb, Cu3Sb, Cu11Sb4, or Cu2Sb.
When indicating these modes, even after heating such as after the silver soldering step/after circuit breaking, the Sb constituent in the contacts is stable and readily remains behind in uniform fashion.
In another preferred mode of the present invention, the mean grain size (if the planar shape is circular, this is the diameter. If it is rectangular, ellipsoidal, or polygonal, it is the diameter calculated as of the circle of that area) of the chemical compound CuxSb referred to above is of grain dimensions 0.02 to 20 xcexcm.
If it is more than the 20 xcexcm, the restriking characteristic is severely impaired and the contact resistance characteristic is also severely impaired. Base material wherein this is less than 0.02 xcexcm is difficult to manufacture economically as a uniform base material. Furthermore, when portions wherein the mean grain size was under 0.02 xcexcm were selected and evaluated, although their contact resistance characteristic showed no abnormality, there was severe variability of their restriking characteristic.
In another preferred mode of the present invention, the mean distance between grains of the chemical compound CuxSb referred to above is highly dispersed, these being isolated by 0.2 to 300 xcexcm.
Isolation of the chemical compound grains by less than 0.2 xcexcm was difficult to achieve with contact manufacturing technology. If they are isolated by more than 300 xcexcm, the CuxSb chemical compound grains tend to aggregate and become of large size, making it difficult to achieve smoothness of the contact surface, due to exfoliation of the chemical compound grains. Also, there is severe variability of the frequency of restriking.
In another preferred mode of the present invention, the mean surface roughness (Rave.(=roughness average)) of the contact surfaces of the contacts referred to above is less than 10 xcexcm, with a minimum value (Rmin.) of at least 0.05 xcexcm.
If the mean surface roughness is more than 10 xcexcm, severe variability of the contact resistance characteristic is seen. Obtaining a contact surface of surface roughness under 0.05 xcexcm presents problems regarding productivity.
In another preferred mode of the present invention, a Cu layer having a thickness of at least 0.3 mm is applied to is the surface on the opposite side to the contact surface of the contacts referred to above.
This facilitates the operation of silver soldering with the electrode and/or conductive shaft.
In another preferred mode of the present invention, surface finishing is performed on the contact surface of the contacts described above by interrupting a current of 1 to 10 mA in a condition with a voltage of at least 10 kV applied.
In a range of 1 to 10 mA, the frequency of occurrence of restriking is greatly diminished. At under 1 mA, no benefit is found. If 10 mA is exceeded, surface irregularity is produced at the contact surface, which has the opposite effect of producing variability of the frequency of occurrence of restriking and variability of the contact resistance.
In general, the arc tends to stagnate and concentrate in regions of low arc voltage. If current interruption is performed whilst applying a magnetic field (for example by the axial magnetic field technique) to the contact, the arc that is generated by the interruption moves over the contact electrode surface instead of stagnating and concentrating in regions of low arc voltage. Transient damage at the contact surface is thereby reduced, improving the interruption characteristic and contributing to a reduction in the probability of restriking. Specifically, since the arc easily moves over the contact electrode, dispersion of the arc is promoted; this is associated with a substantial increase in the area of the contact electrode that is involved in the process of current interruption, thereby contributing to an improvement in the current interruption characteristic. Furthermore, since stagnation and concentration of the arc are reduced, the benefits of prevention of local abnormal evaporation of the contact electrode and reduction of its surface roughness are obtained, contributing to reduction of the probability of restriking.
However, if current of more than a certain value is interrupted, the arc stagnates at one or more points, which cannot be predicted, on the contact surface, causing abnormal melting, and the current interruption limit is reached. Also, the abnormal melting induces instantaneous explosions or evaporation of the contact electrode material, and the metallic vapor that is thereby generated severely impairs insulation recovery of the vacuum circuit breaker in the contact separation step (during contact separation), further lowering the limit of interruption.
Furthermore, the abnormal melting produces giant molten drops, which produce roughness of the contact electrode surface, tending to lower its voltage withstanding ability, increase the probability of occurrence of restriking, and cause abnormal erosion or the material. It is desirable that the contact should be given surface conditions such that the locations of stagnation on the contact electrode surface of the arc which causes occurrence of these phenomena should be completely incapable of being predicted, as described above, and also that the arc generated should be moved and dispersed without stagnation.
Although, as described above, the mechanism of generation of the restriking phenomenon is not known, according to the experimental results of the inventors, restriking occurs with fairly high frequency between one contact and another contact within (inside) the vacuum interrupter, and between the contacts and the arc shield. Accordingly, the inventors were able to achieve a large reduction in the rate of occurrence of restriking by elucidating an extremely effective technique to suppress the generation of restriking by suppressing abrupt gas which is discharged when for example the contacts are subjected to arcing and by promoting optimization of the condition of the contact surface. According to the results of detailed analysis of the aforementioned simulated test of generation of restriking carried out by the inventors in respect of the occurrence of restriking, this was found to be related to cases directly influenced by the contact material, cases influenced by design aspects of the electrode construction and shield construction etc., and external mechanical/electrical conditions such as exposure to unanticipated high voltage. However, it is thought that the limit has been reached in respect of improvement of electrodes as aforementioned in regard to demands for higher voltage withstanding ability, larger current interruption capability, and further miniaturization that are being made in recent years, so some improvement/optimization other than these has become necessary.
As a result of simulated restriking tests conducted by the inventors involving appropriate mounting and removal within the vacuum interrupter of various structural members such as the ceramic insulating container sleeve, contacts, arc shield, metal covers, conductive rod, sealing metal, and bellows, they obtained the discovery that the composition of the contacts that are subjected to direct arcing, their material and condition, and the conditions of their manufacture are vitally important in regard to the rate of occurrence of restriking. In particular, they obtained the discovery that Cuxe2x80x94W or Cuxe2x80x94Mo, which are of high hardness and high melting point, are more advantageous than Cuxe2x80x94Bi, Cuxe2x80x94Te or Cuxe2x80x94Cr alloy, which are observed to display considerable discharge and dispersion of fine metallic particles into the inter-electrode space when subjected to shock as on power-up or interruption, due to the brittle nature of their materials. A further important observational discovery was that, even for the same Cuxe2x80x94W or Cuxe2x80x94Mo, there was variability in regard to the degree of occurrence of discharge and dispersion of fine metallic particles into the inter electrode space, and that, in particular, a high sintering temperature in the process of manufacturing Cuxe2x80x94W or Cuxe2x80x94Mo tended to be beneficial in suppressing occurrence of restriking.
Also, a characteristic feature in the observational results of the inventors regarding the relationship between the time of occurrence of restriking and the material condition of the Cuxe2x80x94W or Cuxe2x80x94Mo was that (a) the contacts composition and their condition (segregation/uniformity) was related to optimization of in particular the mixing conditions of the manufacturing process, and that restriking occurred randomly without regard to the number of times of previous current interruption/switching. (b) A further characteristic feature was that, although the quantity/condition of gas or moisture adhering to or absorbed on the contact surface is a problem of the storage environment (management environment) after processing of the previously finished contacts which does not directly concern sintering technique, restriking is seen from a comparatively early stage in terms of the number of times of current interruption/switching. (c) The importance of the manufacturing process is suggested by the fact that the quality of the raw-material powder (selection of Cu powder, W powder or Mo powder) and the mixing condition of the raw materials are important points in determining the contact interior conditions such as the condition and quantity of impurities incorporated in the interior of the contacts, and it is suggested that these are causes of restriking which occurs comparatively late in terms or number of times of current interruption.
Thus, although the time of occurrence of restriking is apparently unrelated to the history in terms of number of times of current interruption, it was found that the causes thereof differ depending on the time of occurrence as under (a), (b), (c). It is thought that this is an important reason for the manifestation of variability in the occurrence of restriking, between different vacuum interrupters,
Action of the Alloy of the Present Invention
An alloy according to the present invention is constituted by: W (WMo) or Mo (MoW) having the function of improving the mechanical erosion characteristic under interruption power-up operation or switching operation and anti-arcing performance (arc erosion) of the contacts as a whole; Cu (CuSb solid solution) having a function of maintaining a low and stable value of the contact resistance and ensuring conductivity of the contacts as a whole; Ca or CuSb solid solution produced by overheating of W (WMo) or Mo (MoW); and CuxSb chemical compound that bears the function of acting as a restriking stabilization constituent, by mitigating transient evaporation loss of the CuxSb chemical compound. The CuxSb chemical compound functions effectively as a restriking stabilization constituent.
Action (1): in the alloy of the present invention, the content of W (WMo) or Mo (MoW) in the Cuxe2x80x94W alloy, and/or the grain size of W (WMo) or Mo (MoW) is optimized. Furthermore, micro-uniformity of the structure of the contact alloy as a whole is achieved by applying a restriction such that the size of the conductive constituent (Cu phase or CuSb solid solution) surrounded by the W (WMo) or Mo (MoW) is less than 50 xcexcm or the size of less than 50 xcexcm occupies at least a prescribed area. Furthermore, by controlling the grain size of the CuxSb chemical compound to within a range of prescribed values (0.1 to 20 xcexcm), and by controlling the mean distance between grains of the CuxSb chemical compound to within a range of prescribed values (0.2 to 300 xcexcm), the CuxSb chemical compound is put into a highly dispersed condition and the extent of aggregation of CuxSb chemical compound at the contact surface or of its exfoliation from the contact surface is reduced. As a result, the amount of CuxSb chemical compound that is selectively and preferentially evaporated and dispersed on subjection to arcing is restricted to a minimum, the CuxSb chemical compound grains are uniformly distributed at the contact surface, and CuxSb chemical compound constituent in the form of a thin film is uniformly distributed at the contact surface.
Action (2): by controlling the mean grain size of the W (WMo) or Mo (MoW) in the alloy, and the mean grain size of CuxSb chemical compound to practically the same level (size), dispersion and exfoliation of the W (WMo) or Mo (MoW) grains is reduced. Also, wettability between the Cu (CuSb solid solution) and w (WMo) or Mo (MoW) is improved, and adhesion between the W (WMo) or Mo (MoW) grains and Cu (CuSb solid solution) is improved. Furthermore, breaking away of CuxSb chemical compound from the contact surface, which is extremely injurious in regard to occurrence of restriking, even under heat shock during arcing, is suppressed. As a result, stabilization of the restriking characteristic and contact resistance characteristic is achieved.
Action (3): thanks to the control of the condition in which W (WMo) or Mo (MoW) is present, uniformity of the alloy structure is achieved, so, even after arcing, a stable condition of the contact surface in regard to probability of restriking is obtained.
Action (4): as a modified example, it was found that the presence of Mo or W in Cuxe2x80x94W or Cuxe2x80x94Mo is beneficial in reducing discharge and dispersion of fine metallic particles into the inter-electrode space due to snock on power-up or interruption. Normally, on power-up or interruption, breaking-away is observed at the W or Mo surface, and some of this material may be dispersed or exfoliated. Thanks to the presence of Mo or W in the Cuxe2x80x94W or Cuxe2x80x94No, the bonding of the Cu and Mo or Cu and W is strengthened and the plastic deformation capability in extremely small areas is improved.
This is combined with the benefit of controlling the mean grain size of the CuxSb chemical compound and the mean distance between grains referred to above to within prescribed values. As a result, the amount of exfoliated particles produced is itself reduced and even if some exfoliated particles still exist the benefit is obtained of applying a certain degree of rounding at the tips of the scars which they leave. As a result, the electric field concentration coefficient xcex2, which expresses the contact surface condition, is improved from more than 100 to less than 100. This is beneficial in reducing discharge and dispersion of fine metallic particles into the inter-electrode space during interruption. It shows that the CuxSb chemical compound functions effectively as a restriking stabilization constituent. As a result, generation of fine metallic particles by shock on power up or interruption is suppressed to a low level and the amounts of these which are discharged and dispersed become small, contributing to suppression of restriking and contributing to stabilization of the contact resistance characteristic. In this way, they can be simultaneously obtained the benefit of CuxSb chemical compound referred to above having optimized mean grain size and mean distance between the grains, the advantage of improvement of the electric field concentration coefficient xcex2 due to the W (WMo) or Mo (MoW), and a stable contact resistance characteristic and restriking characteristic.
Due to the synergetic effect of these desired actions, with the CuxSb chemical compound in this alloy, while maintaining the current interruption characteristic, a table contact resistance characteristic of the Cuxe2x80x94W or Cuxe2x80x94Mo alloy and suppression of the rate of occurrence of restriking are obtained.